Silicon pressure sensors have broad applications in a multitude of industries. These sensors typically include a thin, flexible diaphragm and a transducer overlaps the diaphragm. Electronic circuitry is employed in the operation of silicon sensors. In an integrated sensor, this other circuitry is disposed on the same chip as the diaphragm. Non-integrated sensors do not have on-board circuitry.
The fabrication of semiconductor devices in general and silicon pressure sensors in particular requires various types of silicon etches employing different etchants. Wet etches are commonly employed in the fabrication of semiconductor devices and KOH (potassium hydroxide) is a well known wet etchant. KOH is an anisotropic etchant that will etch silicon in the {100} plane at a substantially higher rate than it will etch {111} silicon. Further, KOH will etch silicon dioxide while silicon nitride is etch resistant to KOH.
The diaphragm of a pressure sensor is typically formed by etching a cavity into the backside of a silicon wafer a sufficient depth to create the thin diaphragm. KOH is typically used to etch the cavity and a silicon nitride layer is employed as a mask. It is known to cover the edge of the silicon wafer with the silicon nitride mask layer to protect the wafer edge from being etched and also to protect the wafer from other damage during processing. One particular scheme uses a combination of LPCVD and plasma nitride formed over the wafer edge.
There are significant problems associated with utilizing well known etch protection methods. These problems are typically due to failure to compensate for defects in or on the wafer edge. Edge defects are often caused by high temperature processing such as the formation of epitaxial spikes during epitaxial layer growth. Edge damage such as cracks and chips can also be caused while placing wafers in a wafer boat for nitride layer deposition or the like. This can further create undesired particles along the wafer edge.
Nitride layers typically employed for protection are not perfectly conformal relative to defects on the wafer edge and defects are often thicker than the nitride film employed. Since this nitride does not always completely cover the wafer edges, the edges are somewhat exposed to etchant during steps such as the cavity etch. The etch of these exposed portions of the wafer follows high etch rate secondary planes and may cause significant damage. Undesired etching of the wafer edge, in addition to undesireably etching wafer portions, can cause additional pits, cracks and the like that create stress and significantly damage the die being fabricated on the semiconductor wafer, especially those at the wafer edge. This greatly affects product yield.
Another detrimental effect of the well known utilization of nitrides to protect wafer edges is that silicon nitride is generally a very high stress film. The nitride creates stress on the silicon wafer that causes cracks to develop. Stress is also caused in the wafer by epitaxial and buried layers fabricated therein. This may also cause a silicon nitride coating to crack and expose the wafer to etchant.
In view of the above, it would be highly desirable to have a method to protect the wafer edge during semiconductor processing such as the formation of cavities in pressure sensors.